The present invention relates to a bearing assembly for wellbore drilling and, in particular, an oil-sealed bearing assembly for use in a wellbore drilling operation.
In the drilling of bore holes into the earth, as in the case of drilling oil and/or gas wells, it is common to drive the drill bit by a downhole mud motor located at the end of a drill string. In particular, drilling fluid, generally referred to as drill mud, is circulated to drive the motor by positive hydraulic displacement or turbine action. The mud then passes through the ports in the drill bit and carries material loosed by the drill bit back to the surface through the annular space between the drill pipe and the resulting bore hole.
Bearing assemblies for wellbore drilling are mounted between the drill bit and the drill string to permit rotation of the drill bit. The drill bit is attached to a hollow drive shaft, also known as a mandrel that is located within a bearing housing. The mandrel is rotatably driven by the mud motor while the bearing housing is fixed to the drill string and remains relatively stationary. In its position behind the drill bit, the bearing assembly is subject to significant radial and axial loading. Radial and thrust bearings are thus located along the bearing assembly to react (absorb) radial and axial loads.
Lubrication between the rotator mandrel and stator housing may be achieved by oil or mud located in the annular space between those components. In the case of oil lubrication, an oil-sealed bearing chamber is formed by seals such as PolyPak(copyright) or KALSI SEAL(copyright). The seals are acted upon by downhole drilling fluid pressures, including pump pressures and hydrostatic pressures, resulting in higher pressures above the sealed bearing chamber as compared to below the sealed bearing chamber. Such pressure differential results in damage to the seals, leading ultimately to seal failure. To reduce the pressure differential, a flow restrictor located above the sealed chamber is used to reduce the fluid flow in the annular passageway between the mandrel and housing.
In mud lubricated bearing assemblies, it is desirable to reduce the fluid flow through he bearings to prevent premature wear due to the effects of high drilling fluid pressures, as well as the abrasive actions of the mud itself. In this case, radial bearings serve as flow restrictors.
Despite the use of flow restrictors/radial bearings to increase the durability of the bearing assembly, the flow restrictors/radial bearings are themselves limited by their ability to withstand damage resulting from lateral and radial loads incurred during drilling. Carbide-containing flow restrictors are very expensive and damage thereto is of great concern.
The mandrel component of the bearing assembly is also susceptible to damage by drilling loads, as well as by the severe shock and vibration incurred during drilling applications. In particular, the mandrel is engaged to the housing by a split ring, also called a saver ring. The split ring includes two semi-cylindrical halves having annular grooves in their inner surfaces. The machined grooves engage into annular recesses formed on the surface of the mandrel. During assembly, the halves of the split ring are fit over the mandrel. This form of assembly requires that the fit between the mandrel and the split ring to be somewhat loose. This loose fit permits some vibration between the mandrel and the split ring, thereby causing mandrel failure by cracking.
There is a need, therefore, for improved construction of bearing assemblies which provides for a longer operational life of the assembly over current constructions.
A bearing assembly for wellbore drilling has been invented. In one embodiment, the bearing assembly has an extended life without repair by providing support for the flow restrictor/radial bearing (which will be termed herein, the flow restrictor). In another embodiment, a compression nut is threadedly engaged onto the mandrel to replace the split ring.
In accordance with a broad aspect of the present invention there is a bearing assembly comprising: a first elongate member adapted for connection to a drill bit assembly; a second elongate member adapted for connection to a drill string, the first elongate member and the second elongate member being telescopically disposed and being rotatable about a long axis of the bearing assembly one relative to the other; a first bearing disposed between the first elongate member and the second elongate member; a flow restrictor positioned between the first elongate member and the second elongate member to restrict flow between the members; and a second bearing adjacent the flow restrictor and on a side of the flow restrictor opposite the first bearing.
In accordance with another broad aspect of the present invention there is a bearing assembly for use in a drill string to accommodate rotation of the drill bit comprising a first elongate member and a second elongate member, the second elongate member, being telescopically disposed over the first elongate member and the first elongate member being rotatable within the second member and about the long axis of the tool; a flow restrictor disposed between the first elongate member and the second elongate member to restrict fluid flow therebetween; a bearing on each side of the flow restrictor, the bearing assemblies being disposed to act between the first elongate member and the second elongate member.
The first elongate member and the second elongate member can be the mandrel and outer housing of the bearing assembly, respectively.
The bearings are preferably oil-lubricated requiring sealed oil chambers. A sealed, lubricated bearing offers an extended useful life over a mud-lubricated bearing. In a bearing assembly having lubricant-containing sealed bearings, preferably means are provided to balance the seals of the chamber. In one embodiment, at least one fluid flow channel is provided past the upper bearing assembly to provide for balancing the pressures at the seals of the lubricant-filled chamber. In another embodiment, openings are provided through the housing such that fluids at external pressure are in contact with chamber seals.
Preferably, the bearing assembly includes radial and thrust bearings. In one preferred embodiment the thrust bearings include on-bottom bearings for accommodating load in one direction and off-bottom bearings for accommodating load in an opposite direction to the on-bottom bearings. These thrust bearings are positioned on the bearing assembly to be adjusted simultaneously. Preferably, these thrust bearings are contained within the same lubricant-filled chamber.
In a preferred bearing assembly, there are at least three radial bearings spaced along the bearing assembly with, for example, two on one side of the flow restrictor and one on the other side. Preferably, greater radial support is provided between the flow restrictor and the bit end of the bearing assembly where greater lateral and sideways bending loads are experienced. Preferably radial bearings are positioned on either side of the flow restrictor and on either side of the thrust bearings. All radial bearings are preferably selected to maintain the mandrel concentric within the bore of the housing.
In accordance with another aspect of the present invention, there is provided a bearing assembly for use in a drill string to accommodate rotation of the drill bit comprising a first elongate member and a second elongate member, the second elongate member being telescopically disposed over and rotatable about the first elongate member; a flow restrictor disposed between the first elongate member and the second elongate member to restrict fluid flow therebetween; at least one bearing disposed to act between the first elongate member and the second elongate member and a compression nut threadably engaged to the first elongate member and disposed between the first elongate member and the second elongate member, the compression nut abutting against a shoulder on the inner diameter of the second elongate member and supporting the at least one bearing.
Preferably, the bearing is a thrust bearing. In one embodiment, the compression nut is positioned to maintain the second elongate member in telescopic arrangement over the first elongate member.